This invention relates to a method for laser scribing a substrate and, more particularly, to a method of coating a protective material onto a thick-film resistor element after a laser trimming operation and before a laser scribing operation to protect the resistor element during laser scribing.
It is known in the prior art to make electronic circuit products using a ceramic circuit board. One known process for manufacturing a ceramic circuit board is a thick-film process. This process starts with a ceramic plate as the substrate, such as a fired sintered alumina substrate. Conductor paste is first screen printed onto the substrate and fired to form thick-film conductor lines. Thereafter, a pattern of electrical resistance material is screen printed onto the substrate and fired to form thick-film resistor elements which are connected up to the conductor lines. The conductor lines and resistor elements combine to form a plurality of individual thick-film circuits on the substrate.
After the thick-film resistor elements are formed on the substrate, each may be laser trimmed to increase the resistance value thereof. A laser beam projector is employed which produces a laser beam for evaporating a portion of the resistance material of the resistor element, thereby increasing the resistance value thereof.
After the plurality of thick-film circuits are formed on the substrate, they are separated from one another. A laser beam projector which makes scribes in the substrate has been used in the prior art for separating the individual circuits from one another. It has been found, however, that hot-slag ceramic material generated during laser scribing may recondense onto each surface of one or more of the resistor elements. This recondensed material may result in thermal-mechanical damage to a resistor element which may affect the resistance value thereof.
Attempts have been made in the past to avoid damage to resistor elements during a laser scribing operation. For example, laser scribing from the backside of the substrate away from the thick-film circuits has been attempted in the prior art. Such a method, however, has not been found to be acceptable. This is because mechanical damage to the resistor elements, such as scratches, may occur while the substrate is placed in or taken out of a fixture which holds the substrate during scribing. Further, it is difficult to determine on the underside of the substrate the proper location for making each of the scribes.
A further attempt has been made to avoid damage to resistor elements during a laser scribing operation. This attempt involves coating and firing a glass overglaze onto resistor elements. If a glass overglaze is applied and fired onto thick-film resistors, the resistance value of the resistors may change. As a result, the glass overglaze must be applied before the resistor elements are laser trimmed. Laser trimming speed, however, is drastically reduced if a glass overglaze is on the surface of each of the resistors. Further, since the glass remains permanently on the surface of each of the resistors, visual inspection of the resistors and heat dissipation, particularly of power resistors, is also adversely affected.
Accordingly, a need exists for an improved method of laser scribing a substrate having a thick-film circuit with at least one resistor element formed thereon that is rapid and yet does not damage or alter the resistor during the laser scribing operation.